An antigen shared by numerous species of bacterial family may have biologic and clinical significance because (1) it may be used as an indicator for the quantitation of the immune response, (2) it may serve as immunogen for immunization against infection by potential pathogens, and (3) it may function as a pathogenic factor in immunologic disease. The present study is concerned with several common antigens of Enterobacteriaceae, including the common enterobacterial antigen, R core lipopolysaccharide, lipid A, and lipoprotein. Antibodies against these antigens will be quantitated in numerous serum specimens collected during previous studies of the immune response to O antigens of infecting microorganisms from patients with varied enterobacterial infections. Oral immunization of rabbits will be carried out and both the local and humoral antibody response will be identified. Cellular immunity against these antigens will be investigated in immunized guinea pigs using H3-thymidine incorporation into lymphocytes as indicator as well as skin tests. To determine whether these antigens are shared by tissues of several animal species, antisera will be absorbed with cells and cell extracts obtained from cultures. The immunosuppressive effect of lipopolysaccharides will be studied further, utilizing mutants which do or do not produce the common enterobacterial antigen. The genetic control of the production of the immunogenic and non-immunogenic common enterobacterial antigen will be studied by means of well characterized mutants in collaboration with Drs. H. Mayer and G. Schmidt of the Max-Planck Institut fur Immunbiologie, Freiburg, Germany, and Dr. P. H. Makela of the State Serum Institute, Helsinki, Finland.